1. Field of the Invention
The present invention relates to a semiconductor element having a superior weatherproof collecting electrode, and particularly to a photovoltaic element which is easy to fabricate and has long-term waterproofing reliability, as well as a forming method thereof.
2. Related Background Art
In recent years, there has been growing interest in the environment and energy problems such as global heating and radioactive contamination caused by several atomic power plant accidents. In these situations, solar cells using a photovoltaic semiconductor element (hereinafter also referred to as a photoelectric conversion element) have been expected to be used worldwide as a reproducible, inexhaustible clean energy source. At present, three types of solar cells are well known: single crystal silicon type, polycrystalline silicon type, and amorphous silicon type. An amorphous silicon type solar cell is one of the most promising solar cells, because it has excellent characteristics such as easy formation of large area cells and usefulness with thin films due to its great light absorption coefficient, unlike crystalline type solar cells, although it is inferior in conversion efficiency to crystalline type solar cells. Thus, much research on the amorphous silicon type has been made worldwide, because if the amount of photovoltaic power generation reaches several hundreds MW, its cost is estimated to be significantly less than the crystalline types.
An example of a conventional solar cell is shown in FIG. 6. Photoelectric conversion layer 103 made of an amorphous silicon is formed on electrically conductive substrate 104, and transparent electrically conductive layer 102, also useful as an anti-reflection layer, is formed thereon. Further, on the transparent electrically conductive layer there is formed collecting grid electrode 101 for collecting current more effectively. If light is incident on the photoelectric conversion layer 103 from the side of the collecting electrode 101, as shown in FIG. 6, light energy is converted into electric current within the photoelectric conversion layer, and outputted from the collecting electrode 101 and the electrically conductive substrate 104. The photoelectric conversion layer contains at least one or more pin junctions, with the p-side acting as an anode and the n-side as a cathode.
In general, solar cells having an output of several watts or greater are used outdoors. Therefore, these solar cells are required to have durability against temperature and humidity, i.e. be environment-proof.
In particular, when the collecting electrode is formed on the non-single crystal photovoltaic semiconductor layer, it must be formed in a large area at a temperature which does not damage the quality of the film of the semiconductor, whereby an electroconductive paste is used.
In the conventional solar cell shown in FIG. 6, there are a number of voids or interstices 105 of various sizes formed in the collecting electrode 101. In the atmosphere, moisture permeates such voids or interstices and dissolves the electroconductive base substance, such as silver, contained in the collecting electrode due to the photovoltaic EMF of the solar cell itself. Such dissolved electroconductive base substance may diffuse and grow through defective portions such as pinholes in the semiconductor, causing a short-circuit between the positive and negative electrodes of the solar cell and resulting in greatly decreased conversion efficiency. For example, when the electroconductive base substance is silver, a reaction occurs between the anode and the cathode, according to the following formula, thereby giving rise to a short-circuit.
Anode Ag.sub.2 O+H.sub.2 O.fwdarw.2Ag.sup.+ +2OH.sup.- PA1 Cathode Ag.sup.+ +e.sup.- .fwdarw.Ag (dendritic crystal deposition)
This behavior is shown in FIG. 7. Silver ions 605 produced by the reaction between Ag.sub.2 O and water within the collecting electrode 101 of the anode side enter pinhole 606 existing in the photoelectric conversion layer 103 due to the electromotive force produced by the photovoltaic element, adhering to the electrically conductive substrate 104 to form dendritic crystal 607. If the dendritic crystal 607 grows, the collecting electrode 101 and the electrically conductive substrate 104 of the solar cell are short-circuited, thereby decreasing the conversion efficiency. If the reaction further progresses, output of the solar cell cannot be obtained.
This electromigration phenomenon is not limited to silver, but can occur with copper, solder, and gold.
In view of the above-described problem, an object of the present invention is to provide a photovoltaic element which has high environment proofing, and especially is easy to make with conversion efficiency not degrading because of permeation of water.